One technique for offshore drilling and production, particularly in deeper water, utilizes a tension leg platform (“TLP”). A TLP is secured by a number of tendons that attach to pilings in the sea floor. The TLP is de-ballasted to create a desired tension in each of the tendons. The tendons limit lateral movement of the TLP due to waves and currents.
Each of the tendons is preferably close to being neutrally buoyant so that it is substantially self supporting prior to connection to the TLP. Being approximately neutrally buoyant reduces the amount of buoyancy required by the TLP and thus the hull size. To provide buoyancy, the tendons have hollow interiors sealed from sea water. Typically, each tendon is made up of a plurality of joints of pipe, each being approximately 60 to 90 feet in length.
It is important to maintain the buoyancy, because if an interior of one of the tendons filled with sea water, the loss in buoyancy would result in excessive weight being applied to the TLP at the point of connection. It is known to mount sealed bulkheads in the joints of pipe to form separate sealed compartments in the interior of the tendon. Leakage of one compartment would not be as catastrophic as the entire interior of the tendon filling with sea water.
Each tendon must withstand the hydrostatic pressure of the surrounding sea water, which increases with depth. A greater wall thickness will increase the ability of a pipe to withstand hydrostatic pressure. However, a greater wall thickness throughout the length of the tendon would also increase the weight of the tendon, thus requiring a larger and more buoyant hull for the TLP. U.S. Pat. No. 6,851,894 discloses a stepped diameter tendon having upper, intermediate, and lower sections. The upper section has a greater diameter and thinner wall than the intermediate section. Similarly, the intermediate section has a greater diameter and thinner wall than the lower section. This patent does not disclose sealed bulkheads in the interiors of any of the sections.